It is not easy to remove a bubble in highly viscous fluid because of its viscosity, and various defoaming methods have been used. Brief description will be given to the principle and features of the conventional defoaming method described in NPTL 1.
1. Still Stand Method
A solution to be defoamed is left at rest in a large tank or the like. Ascending force arising from the density difference between a bubble and liquid is used as propulsive force and the bubble is removed by natural ascending speed determined by it balanced with the viscous force at the bubble surface.
2. Temperature Rise Method
A non-deforming solution is heated to reduce the viscosity of the solution and increase the natural ascending speed of the bubble.
3. Vacuum Defoaming
A solution to be defoamed is placed under reduced pressure to increase the diameter of a bubble. Then the solution is defoamed by acceleration in the natural ascending speed of the bubble due to increase in ascending force. This method is characterized in that it does not involve temperature rise and is effective to substances and the like that react to heating.
With respect to this, PTL 1 discloses a continuous defoaming device for emulsion utilizing pressure reduction. This continuous defoaming device is made up of a pressure reducing vessel that houses a hollow disk-shaped rotating body comprised of: a circular disk-shaped bottom plate having a concentric lower collision plate so provided that it is in an upright posture or inclined toward the circumferential side; and a top plate having a concentric upper collision plate positioned alternately with the lower collision plate and so provided that it is in a drooping posture or inclined toward the circumferential side and supported by the bottom plate. The pressure reducing vessel includes: a pipeline for supplying liquid containing a bubble to the central part of the disk-shaped rotating body; and a pressure reducing means.
PTL 2 discloses a continuous defoaming device for bubbles mixed in paint on a roll. This continuous defoaming device is a continuous defoaming device configured to remove bubbles mixed in paint in a vacuum chamber. It includes: a defoaming roll means comprised of a roll for transferring paint from the atmosphere to the vacuum chamber, a roll for discharging paint from the vacuum chamber into the atmosphere, and a group of rolls for vacuum sealing the area between these rolls and the vacuum chamber wall; and a paint recovery means for recovering defoamed paint on the roll for discharging paint from the vacuum chamber.
4. Centrifugal Defoaming by Planetary Stirring
This is a defoaming method utilizing the density difference between gas and liquid. In centrifugal defoaming by planetary stirring, a bubble placed in a vessel is caused to collide with the inner wall of the vessel by centrifugal force to crash it. The crushed bubble is dissolved in a solution and is thereby removed. With respect to continuous centrifugal defoaming that does not use planetary stirring, the following two patents are published:
PTL 3 relates to a multistage centrifugal defoaming device. Several rotary cylinders that exert centrifugal force on liquid are provided and a ring-shape scattering interception component is provided thereabove. Scattering liquid material is once received and centrifugal defoaming is thereby sequentially and repeatedly carried out. The patent literature also describes that it can also be incorporated in a manufacturing line or the like and continuously operated by continuously putting in liquid material from an injection port and continuously recovering the defoamed liquid.
PTL 4 relates to a continuous defoaming device. A rotary cylinder rotated by a motor is placed in a vacuum vessel. Liquid material is injected thereinto from an injection port by an injection pump and the liquid material is caused to fly from the upper part of the rotary cylinder toward the wall surface of the vacuum vessel by centrifugal force. Accumulation of the defoamed liquid material at the lower part of the vacuum vessel is detected by a liquid level sensor and it is discharged from a discharge port by a discharge pump. The patent literature describes that continuous defoaming can be carried out automatically regardless of the initial rate of bubble content by taking the following measure: the liquid level sensor is provided and the discharge pump is controlled from a control panel so that the liquid level of the defoamed liquid material accumulated at the lower part of the vacuum vessel is within an appropriate range.
PTL 5 and NPTL 2 disclose bubble removal methods using a vessel (closed vessel) so structured that its volumetric capacity can be varied. In these methods, a viscous fluid containing a bubble is sealed in the vessel and the volumetric capacity of the vessel is varied using vibration. The interior of the vessel is thereby alternately depressurized and pressurized to increase and reduce the volume of the bubble and remove the bubble.
In these conventional arts, the viscous fluid is an incompressive fluid. Since the volume of a bubble is varied according to pressure, the volume of the bubble is increased and reduced by alternately depressurizing and pressuring the interior of the vessel. As the result of the volume of the bubble being increased and reduced, a shear flow is produced around the bubble.
The viscous fluid used in these conventional arts has a property that its apparent viscosity is reduced by shear. For this reason, when a shear flow is produced around a bubble, the apparent viscosity of the viscous fluid in proximity to the bubble is reduced. Thus the viscosity resistance against the natural ascent of the bubble is reduced and the natural ascending speed of the bubble is increased. As a result, the bubble can be removed from the viscous fluid.